Process for removing impurities from zone refined materials

ABSTRACT

Process and apparatus for increasing the purity levels in zone refined materials, which process includes making one or more zone refining passes in an ingot located in a first, zone refining boat and transferring a portion of the molten zone from the first boat into a second, adjacent boat. The first boat is joined to the second boat by a spillway whose heighth relative to that of the adjacent molten zone is such that controlled amounts of molten material in the zone may pass over the spillway into the second boat at the end of a zone refining pass.

United States Patent [1 1 Cole et al.

[451 Sept. so, 1975 PROCESS FOR REMOVING IMPURITIES FROM ZONE REFINED MATERIALS [73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: Dec. 20, 1972 [21] Appl. No.: 317,505

Primary E.\'aminerAllen E. Curtis Assistant Examiner-Thomas A. Waltz Attorney, Agent, or FirmWilliam J. Bethurum; W. H. MacAllister 5 7 ABSTRACT Process and apparatus for increasing the purity levels in zone refined materials, which process includes making one or more zone refining passes in an ingot located in a first, zone refining boat and transferring a portion of the molten zone from the first boat into a second, adjacent boat. The first boat is joined to the second boat by a spillway whose heighth relative to that of the adjacent molten zone is such that controlled amounts of molten material in the zone may pass over the spillway into the second boat at the end of a zone refining pass.

3 Claims, 2 Drawing Figures US. Patent Sept. 30,1975

Fig. 2.

PROCESS FOR REMOVING IMPURITIES FROM ZONE REFINED MATERIALS FIELD OF THE INVENTION This invention relates generally to the zone refining of high purity materials and particularly to the elimination of undesirable impurities in semiconductors during the zone refining thereof.

BACKGROUND It is well known in the electronic materials art that the impurity density in single crystal ingots of semiconductive material may be reduced by establishing a molten zone in such material and then moving the zone along the material one or more times at controlled rates and temperatures. These materials include silicon, germanium, group Ill-V compounds such as gallium arsenide, or the group IIVI compounds such as cadmium telluride. During this zone refining process, the molten zone produces continuous melting and recrystallization of the ingot as is also well known. This process has the effect of increasing the purity of the ingot by, among other things, moving the randomly distributed impurities in the ingot toward one or the other end of the ingot, depending on the segregation coefficient of the crystal. As a result of the movement of the liquid-solid interface along the length of a semiconductor ingot, these impurities are swept toward one end of the ingot where they build up in quantity asymtotically with time during a number of successive zone refining passes.

In order to achieve the very high purity required in processing certain electronic grade zone refined semiconductive materials, it becomes necessary to physically remove these impurities from one end of the semiconductor ingot at some time during a zone refining operation. As long as these impurities remain in one end of the semiconductor ingot, there is a limit to the amount of additional impurities which can be swept by the molten zone out ofthe main body of the semiconductor ingot and into the saturated or semi-saturated end thereof.

PRIOR'ART Hitherto, the standard procedure for removing these impurities from zone refined materials during a zone refining operation involved the following process steps of: (l) shutting down the zone refining operation after one or more zone refining passes; (2) cooling the semiconductor ingot sufficiently for handling; (3) removing the semiconductor ingot from the zone refining boat; (4) cropping off a portion of one end of the ingot to thereby remove some of the impurities therefrom; (5) returning the ingot to the zone refining boat; and (6) starting up the zone refining operation all over again. Each of the above steps is time consuming and increases the probability of introducing impurities into the ingot either by handling or atmospheric contact to the ingot or both. These steps also add to the overall cost of the zone refining process and to the total time required to produce semiconductor ingots of high purity. Additionally, the cropped-off end of the ingot is wasted material, which obviously also adds to the time and cost of producing high purity zone refined materials.

THE INVENTION The general purpose of thisinvention is to provide a process and apparatus for zone refining electronic materials and for achieving the same high purity as is achieved using the above prior art process, but doing so in less time and with greater yields that the above prior art process. At the same time, the present process has eliminated all of the above six steps of the prior art process. To attain this purpose, the present invention utilizes a novel dual boat structure in carrying out a novel spill-over process for periodically removing impurities from a zone refined ingot. This dual boat structure includes first and second boats which are integrally joined by a spillway, the heighth of which allows a controlled amount of zone refined material to pass from a molten zone in the first boat into the second boat at the end of each zone refining pass. In this manner, impurities are periodically and controllably removed from the zone refined ingot during a zone refining operation without the necessity for the shutdown and start-up operations required by the above-described prior art process.

It is therefore a general object of this invention to provide a new, improved and economical process and apparatus for producing high purity zone refined elec tronic materials.

Another object is to provide a process of the type described which, in comparison to known prior zone refining techniques, may be rapidly carried to completion at high yields.

Another object of this invention is to provide a process of the type described in which impurities may be efficiently removed from a zone refined ingot with a minimum of time and effort on the part of an operatorv A further object of this invention is to provide a process of the type described which increases the yields of high purity zone refined semiconductive materialsv DRAWING FIG. 1 is an isometric view of the dual boat structure according to the invention; and

FIG. 2 is a side elevational view of FIG. 1.

GENERAL PROCESS AND APPARATUS DESCRIPTION Referring now to FIG. 1, there is shown a dual-boat container, designated generally I0, including a first boat 12 and a second boat 14 which are integrally joined by a spillway I6. Advantageously, the container 10 is fabricated of either carbon or quartz and does not chemically react with the materials processed therein. The first boat 12 is of a suitable elongated size and shape for carrying an ingot 18 of electronic material to be zone refined.

A single crystal seed 20, which is of the same contour as the end of the boat 10, is placed as shown in the far lefthand end of the boat 10. This Contour prevents any molten material from running under the seed. The seed 20 is long enough so as to allow a good melt-in into the molten zone 22 without being drawn'into the molten zone 22. After the ingot 18 is placed in the position shown in the first boat 12, it is melted using conven tional and well-known resistance or RF heating techniques to form a molten zone 22 at the lefthand end of the ingot. The molten zone 22 is initially not in contact with the seed 20, but is later brought into contact with the seed crystal 20 as shown in FIG. l. Thereafter the molten zone 22 is traversed one or more times along the length of the ingot 18in a standard conventional zone refining operation. This traverse may be accomplished by providing relative motion between the boat and a surrounding heater coil (not shown) which controllaby melts the zone 22. During the initial zone refining pass, the crystal structure of the ingot 18 is converted to that of the seed 20 in accordance with well-known crystal growth processes.

During a zone refining pass, the molten zone 22 is swept to the right hand end of the first boat 12 as shown in FIG. 2, and at this time controlledamounts of the molten ingot material are passed from the zone 22 and over the spillway 16 into the second boat 14. The heighth of the spillway 16 is chosen such that when the leading edge of the molten zone 22 is'swept to the extreme right hand portion of the first boat 12 and makes contact with the spillway 16, the heighth of the zone 22 is slightly less than the heighth of the spillway 16 at the center of spillway. By urging the zone 22 against the spillway 16 by a further movement of the heater coil, a small controlled amount 24 of the molten zone 22 is caused to flow over the central portion of spillway 16 and into the second boat 14. This amount may be increased or decreased by increasing or: decreasing the momentum of the molten zone 22 when it reaches the spillway. The exact height of the spillway is not critical, since the amount of zone spillover may be controlled by controlling the degree of tilt of the entire boat. Also,

the surface tension at the edge of the spillway 16 which contacts the zone will tend to inhibit the uncontrolled spillover of the molten zone and thereby add a further dimension of control to the process.

The present process is applicable for the zone refining of electronic materials having a segregation coefficient, K ,'less than unity. K is expressed as impurity concentration in the solid Impurity Type Segregation Impurity in lnSb Coefficient Sulphur N 0.16 Selenium N 0.35 Tellurium N 0.5 1.0 Tin N 0.06 Germanium P Silicon P Cadmium P 0.2 Copper P 7 X 10* Silver P 5 X 10- Gold P 2 X 10- However, if K l, the impurities will stay in the solid and will tend to move toward the seed end of the ingot l8 after'a given number of zonerefining passes.

Thepresent invention has the further advantage over the above prior art method in that the spilled over zone refined material 24 is reclaimable and may be used in subsequent zone refining passes. However, since the impurity concentration is high in this material 24, more zone refining passes will be required in a subsequent refining operation in order to attain desired levels of purity in such material.

The following description is a specific example of a process according to the invention which has been successfully reduced to practice. In this example, the starting materials were elemental In and Sb rather than the precompounded lnSb ingot 18 described above. Therefore, it is to be understood that the present invention covers either the zone refining method of starting with a precompounded ingot 18 or the method of melting in the individual elements of the compound material.

' EXAMPLE The dual-boat container 10 was initially coated inside with pyrolytic silicon dioxide for preventing the adherence thereto of thezone refined materials. Next, stoichiometri c amounts of indium and antimony, i.e. gm In, l55.5' gm Sb, were placed close to but separate from a seed crystal which was placed at the lefthand end of the first boat 12. The loaded dual-boat container 10 was then transferred to a closed tube inside a zone refining furnace (not shown). The tube was then sealed off and flushed witha high purity inert gas, which in the present example was hydrogen. The zone refining furnace temperature was then raised until the ingottemperat'ure reached approximately 530C in order to initially completely melt the In and Sb and form the molten ingot material 18. At the same time none of the molten ingot material was permitted to contact the seed "crystal 20. After the ingot material 18 was com pletely molten, it was brought into very gradual contact with the seed crystal 20, melting a small portion of the seed. Then a molten zo'newas maintained adjacent to the seed 22 while the remainer of the ingot compound material 18 was cooled and reverted to a solid polycrystalline state. The molten zone 22 was then moved along the length of the lnSb ingot .18 until reaching the spillway 16, at which time a small portion of this zone 22 was caused to pass over the-spillway 16 into the second boat 14. The boat 10 was approximately 40 mm in diameter and the lnSb rose about A: inch over the height of the spillway 16.as the zone was swept into contact therewith. This height will change with different material used-due todifferences in their surface tensions; and-the amount of spillover can be controlled by either controlling the zone refining temperature, or the tilt of the boat, or both.

After making 20 such zone refining passes, the furnace melt-inpoint. was moved a short distance nearer the spillway 16 from the original melt-in point and the above zone refining passes were continued. This procedure should be repeated as many. times as is determined necessary, which in the present example was determined to be between 3 and 5 times for the levels of purity desired. This repetitive procedure is very important in order to obtain a high purity level near the nose or seed end of the ingot 18. Onlya few drops of the molten zone 22 were passed over the spillway 16 for each of the above zone refining passes.

Next, the heater in the zone refining furnace was turned off and the above apparatus was allowed to cool. After cooling, the closed tube was opened and the dual boat container was removed therefrom. The indium antimonide ingot was carefully removed from the boat 12 under very clean conditions, and the nose or the lefthand seeded section of the ingot was cropped off at a distance from the last melt-in.

A comparison of the purities of zone refining semiconductive materials using the new dual boat inventive process and apparatus versus those achieved using the prior art single boat method is given as follows.

The carrier concentration and mobility are wellknown purity measurements to those skilled in the art. It will be appreciated by those skilled in the art that the present apparatus and process are not restricted to the zone refining of a particular type of electronic material, with the above exception that K 1.

What is claimed is:

l. A zone refining process for removing impurities from an elongated ingot having a segregation coefficient less than one, which process includes the steps of:

a placing said ingot in a zone refining boat;

b. creating a molten zone in a selected region of said ingot;

c. moving said molten zone along the length of said ingot one or more times during which time the actual molten material in said ingot is continuously changing, as solid material at the leading edge of the moving zone is melting, and the molten material at the trailing edge of the moving zone is resolidifying, and, during which time impurities build up at one end of said ingot; and

d. passing a portion of said molten zone over a spill way at one end of the zone refining boat to thereby isolate some of the impurities from said ingot and result in an ingot having a high purity level and eliminate the necessity for removing said ingot from the boat and cropping off the end of said ingot before said zone refining process is completed.

2. The process defined in claim 1 wherein said material is electronic grade semiconductive material.

3. The process defined in claim 2 wherein said material is indium antimonide and said impurities are within a group consisting of sulphur, selenium, tellurium, tin,

germanium, silicon, cadmium, copper, silver and gold. 

1. A ZONE REFINING PROCESS FOR REMOVING IMPURITIES FROM AN ELONGATED INGOT HAVING A SEGREGATION COEFFICIECT LESS THAN ONE, WHICH PROCESS INCLUDES THE STEPS OF: A. PLACING SAID INGOT IN A ZONE REFINING BOAT, B. CREATING A MOLTEN ZONE IN A SELECTED REGION OF SAID INGOT, C. MOVING SAID MOLTEN ZONE ALONG THE LENGTH OF SAID INGOT ONE MORE TIMES DURING WHICH TIME THE ACTUAL MOLTEN MATERIAL IN SAID INGOT IS CONTINUOUSLY CHANGING, AS SOLID MATERIAL AT THE LEADING EDGE OF THE MOVING ZONE IS MELTING, AND THE MOLTEN MATERIAL AT THE TRAILING EDGE OF THE MOVING ZONE IS RE-SOLIDIFYING, AND, DURING WHICH TIME IMPURITIES BUILD UP TO ONE OF SAID INGOT, AND D. PASSING A PORTION OF SAID MOLTEN ZONE OVER A SPILLWAY AT ONE END OF THE ZONE REFINING BOAT TO THEREBY ISOLATE SOME OF THE IMPURITIES FROM SAID INGOT AND RESULT IN AN INGOT HAVING A HIGH PURITY LEVEL AND ELIMINATE THE NECESSITY FOR REMOVING SAID INGOT FROM THE BOAT AND CROPPING OFF THE END OF SAID INGOT BEFORE SAID ZONE REFINING PROCESS IS COMPLETED.
 2. The process defined in claim 1 wherein said material is electronic grade semiconductive material.
 3. The process defined in claim 2 wherein said material is indium antimonide and said impurities are within a group consisting of sulphur, selenium, tellurium, tin, germanium, silicon, cadmium, copper, silver and gold. 